1. Field of the Invention
This invention relates to the field of force compensating apparatus, for example for offsetting the weight of a vertically movable body such as a window sash, wherein a coiled leaf spring, also known as a clock spring or "power spring," supplies a compensating force to render the body subject to the force more easily positioned within its range of movement. More particularly the invention relates to a weight compensating apparatus of this type wherein a nonlinear pulley cancels the nonlinearity of spring force vs. spring deflection that occurs due to the specific nature of the power spring.
2. Prior Art
It is well known to at least partially offset the weight of a movable body which is to be raised and lowered within a range of movement. If precisely balanced, the body remains in place when positioned, rather than tending to fall. An operator or the like need only exert the force needed to move the body from one position to another, and need not offset the weight of the body itself. To avoid the need for accommodating a counterbalancing deadweight, the weight of the body is advantageously offset with a spring. Familiar examples of spring counterbalanced weights are window sashes, garage doors, movable-panel blackboards, and supporting carriages for vertically movable equipment.
Spring force varies as a function of deflection of the spring; however, the weight of the movable body is fixed. In order to compensate for the additional force exerted by a counterbalancing spring when the spring is deflected (e.g., as a window sash is being lowered), it is known to provide a compensating pulley that converts a linearly varying spring force to a constant force. A flexible cord, cable, wire, rope or the like connects the spring and the supported weight via a conical or spiral pulley. The spiral pulley defines a radius for the cord which varies linearly with displacement of the ends of the spring, and thus defines a linearly varying moment arm to compensate for the variation in spring force over the range of movement of the body. This idea is workable for various types of springs in extension or retraction, and for various weights and other constant force exerting conditions.
The spring and compensating apparatus used to offset the weight of a window sash or the like is typically mounted in either the window frame or in the window sash, with the flexible cord attached to the other of the frame and the sash. One difficulty encountered is that the structure of the sash balance apparatus including the spring and compensating pulley, together with additional pulleys needed to route the cord along its path, can be rather large. Another difficulty is with wear on the cord and pulleys. The varying forces are such that the connecting cord and/or the pulley arrangement may wear substantially in the areas of contact at the ends of the range of deflection of the sash or other movable body. In U.S. Pat. No. 4,914,780--Rogers et al, additional elements are provided along the force transmission path, including a constant radius (cylindrical) pulley and a block and tackle force reduction mechanism, to address this problem. Such additional elements further increase the dimensions of the balance apparatus which must be housed in the window frame and/or the sash.
Rogers et al teach using a helical coil spring for producing force varying linearly with spring deflection, namely compression and extension of the spring along the central axis of the helix. This form of spring obviously requires a housing at least as long as the full extension length of the spring. A more compact form of spring is possible, wherein the spring is wound spirally in a plane. The inner end of the spring can be fixed, and the outer end can be rotated around the fixed inner end or pulled outwardly along a tangent. Similarly, the outer end can be fixed and the inner end arranged to rotate a shaft. The movable end of the spring is connected to rotate a conical pulley relative to the fixed end. This spiral form of spring, known as a power spring or clock spring, is relatively compact.
Weight balancing window sash apparatus comprising one or more power springs, i.e., springs wound spirally in a plane, and also including a conical pulley arrangement, are disclosed for example in U.S. Pat. Nos. 4,012,008--Hosooka; 1,599,872--Braen; 550,650--Smelser; 221,247--Milner; 145,289--Faries; 132,631--Chance; and, 97,263--Anderson. The spring is typically a wound steel strip or leaf, fixed at its center to the sash or the frame in which the sash is moved. The outer end of the spring is attached to a rotatable drum and a conical pulley is either attached coaxially to the drum or defined by the outer surface of the drum. As the spring is wound (or unwound), the cord is extracted from (or wound onto) the conical pulley. The point of tangent contact between the cord and the conical pulley varies axially along the conical pulley with relative rotation of the spring ends, and accordingly the effective radius of the conical pulley varies as well. The object is to provide a linearly varying moment arm by means of the conical pulley to precisely counter the varying force exerted by the spring as the spring is wound or unwound and to produce a constant force for offsetting the constant weight of the sash.
It is axiomatic that the force exerted by a resilient structure defining a spring in extension or compression varies linearly with the relative displacement of the ends of the spring. The same is true of torsional (twisting) displacement of the ends of a resilient body. According to the foregoing patents, wherein the pulley carrying the cord is conical, a linear relationship or "spring constant" is assumed for the spring as a whole, with the spring force compensated in an amount directly proportional to displacement.
However, in a power spring, the force exerted by the spring as a whole is not linear, i.e., not directly proportional to displacement. The effectiveness of known power spring balances is thus limited. Various modifications of power spring apparatus have been suggested to improve operation, but persons skilled in the art have continued to assume that a power spring should be compensated in the same manner as an extension spring.